1. Field of the Invention
This invention relates to a two-wire transmitter having a starter circuit.
2. Description of the Related Art
A two-wire transmitter in a related art generally has a starter circuit made of a time constant circuit. (For example, JP-B-3057650) At the start time, the starter circuit secures power supply and improves the stability of the transmitter. The configuration of such a two-wire transmitter in a related art will be discussed with FIG. 2. FIG. 2 is a diagram to show the configuration of the two-wire transmitter in the related art.
The configuration shown in FIG. 2 will be discussed. A two-wire transmitter 10′ is connected to transmission lines L1 and L2. The two-wire transmitter 10′ and the transmission lines L1 and L2 are part of a smart or a fieldbus (Fieldbus Foundation, Profibus (Trademark)).
The two-wire transmitter 10′ includes current control section (30, R3) made up of a current control circuit 30 and a resistor R3 of current detection section. More particularly, for example, one end of the current control circuit 30 is connected to the transmission line L1 and an opposite end of the current control circuit 30 is connected to an internal circuit 50 through power supply section 40. Further, one end of the resistor R3 is connected, for example, to the transmission line L2 and the current control circuit 30 and an opposite end of the resistor R3 is connected to a common potential COM, the current control circuit 30, and the power supply section 40.
The two-wire transmitter 10′ includes the power supply section 40 formed of a Zener diode, for example, and the internal circuit 50 having a sensor (not shown), a microprocessor (not shown), etc. More particularly, an anode of the Zener diode of the power supply section 40 is connected to the common potential COM and a cathode of the Zener diode of the power supply section 40 is connected to the opposite end of the current control circuit 30 (voltage Vcc). The microprocessor (not shown) of the internal circuit 50 is connected the power supply section 40 through a separate regulator (not shown).
In the configuration shown in FIG. 2, the power supply section 40 is formed of the Zener diode, but can also be formed using an error amplifier (not shown), etc.
The two-wire transmitter 10′ further includes a starter circuit 20′. The internal configuration of the starter circuit 20′ will be discussed below: A drain of an n channel depletion type field effect transistor (FET) Q4 is connected to the one end of the current control circuit 30 (transmission line L1) and a source of the FET Q4 is connected to the opposite end of the current control circuit 30.
A series circuit of resistors R5 and R6 is connected between the opposite end of the current control circuit 30 (voltage Vcc) and the common potential COM. Further, a resistor R7 is connected between the connection point of the resistors R5 and R6 and a gate of the FET Q4. A capacitor C is connected between the opposite end of the current control circuit 30 (voltage Vcc) and the gate of the FET Q4. The resistors R5, R6, and R7 and the capacitor C make up a time constant circuit.
Further, voltage at the connection point of the opposite end of the current control circuit 30, the source of the FET Q4, the resistor R5, the capacitor C, and the power supply section 40 is voltage Vcc; voltage at the connection point of the transmission Line 2, the current control circuit 30, and the resistor R3 is voltage Va; voltage at the connection point of the transmission Line 1, the one end of the current control circuit 30, and the drain of the FET Q4 is voltage Vb; voltage at the connection point of the resistors R5, R6, and R7 is voltage Vf; and voltage at the connection point of the gate of the FET Q4, the capacitor C, and the resistor R7 is voltage Vg. The common potential COM is connected to the resistors R3 and R6, the power supply section 40, the current control circuit 30, the internal circuit 50, etc.
The stationary operation of the two-wire transmitter (starter circuit) in the related art in FIG. 2 is as follows:
At this time, the FET Q4 is turned off. More particularly, since the voltage Vcc is sufficiently large, the voltage Vf becomes sufficiently large, the voltage Vg becomes sufficiently large, and the gate-source voltage of the FET Q4 (Vg−Vcc) becomes equal to or less than cutoff voltage.
Power is supplied to the two-wire transmitter 10′ via the transmission lines L1 and L2. The power supply section 40 generates a predetermined voltage and supplies power to the current control circuit 30, the internal circuit 50, etc. Further, the sensor (not shown) of the internal circuit 50 generates an electric signal responsive to the detected physical quantity, and the microprocessor (not shown) of the internal circuit 50 processes the electric signal from the sensor and transmits the processed signal to the current control circuit 30.
The voltage Va proportional to the transmission current flowing into the transmission lines L1 and L2 occurs on the resistor R3. The current control circuit 30 controls so that the electric signal responsive to the detected physical quantity and the voltage Va correlate to each other.
Consequently, the current flowing into the resistor R3 becomes the current responsive to the detected physical quantity and the transmission current flowing into the transmission lines L1 and L2 also becomes the current responsive to the detected physical quantity. The two-wire transmitter 10′ thus transmits the electric signal regarding the detected physical quantity to the transmission lines L1 and L2.
Next, the operation the two-wire transmitter in the related art in FIG. 2 at the starting time is as follows: Before starting, the gate of the FET Q4 (voltage vg) is pulled down with the resistors R7 and R6 and the capacitor C discharges with the resistors R7 and R5. The FET Q4, which is of depletion type, is turned on. At the starting time, as the voltage Vb rises, drain current Ist′ of the FET Q4 flows. The drain current Ist′ promotes the rise in the voltage Vcc. Further, the voltage Vcc rises, the voltage Vg rises, and the FET Q4 continues on.
Then, the voltage Vg lowers having the time constant of the resistors R5, R6, and R7 and the capacitor C, the gate-source voltage of the FET Q4 (Vg−Vcc) lowers and becomes equal to or less than the cutoff voltage, and the FET Q4 is turned off.
The two-wire transmitter (starter circuit) in the related art in FIG. 2 thus provides a stable start characteristic. When the starter circuit does not operate at the starting time, shortage of power supply occurs, the voltage Vcc lowers, and the two-wire transmitter in the related art in FIG. 2 may become unstable.
Japanese Patent No. 3057650 (JP-B-3057650) is referred to as a related art.
With the two-wire transmitter in the related art, however, if the voltage Vcc changes very slowly, namely, if an external power supply (not shown) connected to the transmission lines L1 and L2 starts slowly, whether or not reliable starting can be conducted is unknown; this is a problem. At this time, the time constant circuit made up of the resistors R5, R6, and R7 and the capacitor C does not function effectively.